Orest Pizio

Phase behavior of ionic fluids in slitlike pores: A density functional approach for the restricted primitive model

We present a density functional theory of nonuniform ionic fluids. This theory is based on the application of the electrostatic contribution to the free energy functional arising from mean spherical approximation for a bulk restricted primitive model and from the energy route bulk equation of state. In order to employ this functional we define a reference fluid and additional averaged densities, according to the approach introduced by Gillespie, Nonner and Eisenberg [J. Phys.: Condens. Matter 14, 12129 (2002)]. In the case of bulk systems the proposed theory reduces to the mean spherical approximation equation of state, arising from the energy route and thus it predicts the first-order phase transition. We use this theory to investigate the effects of confinement on the liquid-vapor equilibria. Two cases are considered, namely an electrolyte confined to the pore with uncharged walls and with charged walls. The dependence of the capillary evaporation diagrams on the pore width and on the electrostatic potential is determined.

Evaluation of liquid–vapor density profiles for associating fluids in pores from density-functional theory

Using density-functional theory we calculate density profiles of an associating fluid in slit like pores as functions of two variables: The distance from the pore wall and the distance along the pore axis. Attention is focused on evaluation of the profiles characterizing the coexistence between two confined phases. We also calculate changes in the grand canonical potential connected with the formation of an interface between two coexisting confined phases. Specific calculations have been carried out for the associating, chain forming Lennard-Jones fluid adsorbed in a slitlike pore.

Understanding the structure of aqueous cesium chloride solutions by combining diffraction experiments, molecular dynamics simulations, and reverse Monte Carlo modeling.

A detailed study of the microscopic structure of an electrolyte solution, cesium chloride (CsCl) in water, is presented. For revealing the influence of salt concentration on the structure, CsCl solutions at concentrations of 1.5, 7.5, and 15 mol % are investigated. For each concentration, we combine total scattering structure factors from neutron and X-ray diffraction and 10 partial radial distribution functions from molecular dynamics simulations in one single structural model, generated by reverse Monte Carlo modeling. This combination of computer modeling methods is capable of (a) showing the extent to which simulation results are consistent with experimental diffraction data and (b) tracking down distribution functions in computer simulation that are the least comfortable with diffraction data. For the present solutions, we show that the level of consistency between simulations that use simple pair potentials and experimental structure factors is nearly quantitative. Remaining inconsistencies seem to be caused by water-water distribution functions. Changing the pair potentials of water-water interactions from SPC/E to TIP4P-2005 has not had any effect in this respect. As a final result, we obtained particle configurations from reverse Monte Carlo modeling that were in quantitative agreement with both diffraction data and most of the molecular dynamics (MD) simulated partial radial distribution functions (prdfs). From the particle coordinates, the distribution of the number of first neighbors, as well as angular correlation functions, were calculated. The average number of water molecules around cations decreases from about 8 to about 6.5 as concentration increases from 1.5 to 15 mol %, whereas the same quantity for the anions changes from about 7 to about 5. It was also found that the average angle of Cl...H-O particle arrangements, characteristic of anion-water hydrogen bonds, is closer to 180 degrees than that found for O...H-O arrangements (water-water hydrogen bonds). The present combination of experimental and computer simulation methods appears to be promising for the study of other electrolyte solutions.

Comparison of interaction potentials of liquid water with respect to their consistency with neutron diffraction data of pure heavy water

A number of interaction potential models for liquid water are scrutinized from the point of view of their compatibility with results of neutron diffraction experiments on pure heavy water. For the quantitative assessment a protocol developed recently [L. Pusztai et al., Chem. Phys. Lett. 457, 96 (2008)] using the reverse Monte Carlo method has been applied. The approach combines the experimental total scattering structure factor (tssf) and partial radial distribution functions (prdfs) from molecular dynamics simulations in a single structural model (particle configuration). Goodness-of-fit values to the three (O-O, O-H, and H-H) simulated prdfs and to the experimental tssf provided an unbiased measure characterizing the level of consistency between various interaction potentials and diffraction experiments. Out of the sets of prdfs investigated here, corresponding to SPCE, BJH, ST2, POL3, TIP4P, TIP4P-2005, TTMF3, and ENCS interaction potentials, the ones from the TIP4P-2005 potential proved to be the most consistent with the experimental neutron-weighted tssf of heavy water. More importantly, it is shown that none of the above interaction potentials are seriously inconsistent with the measured structure factor at ambient conditions.

ASSOCIATIVE REPLICA ORNSTEIN-ZERNIKE EQUATIONS AND THE STRUCTURE OF CHEMICALLY REACTING FLUIDS IN POROUS MEDIA

A model for a chemically associating fluid, adsorbed in a disordered porous media, is proposed. The formation of the associates occurs through the directional bonding between the fluid particles. For simplicity, we restrict our attention to the dimerization of particles. In the absence of association, this model reduces to that of Kaminsky and Monson (KM) for the adsorption of methane in a xerosilica gel. This model is studied by means of the replica Ornstein–Zernike ROZ equations, with the hypernetted chain approximation, extended for associating fluids. It follows from a comparison with the computer simulation data that this theory yields a very good description of the structural properties of the KM model. The influence of the fluid density, the matrix packing fraction, and the association energy on the dimerization in the disordered matrix is studied. The fluid compressibility for the KM model and for the dimerizing fluid in a disordered matrix is obtained via the compressibility equation.

Density-functional theory for fluid mixtures of charged chain particles and spherical counterions in contact with charged hard wall: Adsorption, double layer capacitance, and the point of zero charge

We consider a density-functional theory to describe nonuniform fluids composed of chain molecules, containing a charged segment each, and spherical counterions. The chain molecules are modeled as freely jointed chains of hard spheres, the counterions are oppositely charged spheres of the same diameter as all segments of chain molecules. The theory is applied to study the structure of adsorbed layers, the excess adsorption isotherms, the capacitance of the double layer, and the potential of the zero charge. We show that all electric properties are strongly dependent on the length of the chain molecules. Moreover, these properties are also dependent on the position of the charged segment in the chain.

DENSITY PROFILES OF A MODEL OF ASSOCIATING HARD SPHERES IN CONTACT WITH A CRYSTALLINE SURFACE : AN INTEGRAL EQUATION APPROACH

Density profiles of a fluid of associating or chemically reacting hard spheres near a crystalline surface are studied. The model of Cummings and Stell is utilized to provide the description of a bulk associating fluid. The crystal symmetry of the substrate surface plane is assumed to be that of the (100) plane of the face centered cubic lattice. The model of the particle–solid interaction is that proposed by Steele. The effect of association of the particles of the bulk fluid on the density profiles of particles near the crystalline surface is investigated within the hypernetted chain approximation. When the first layer is completed, some dimers tend to be vertically oriented over the adsorption site.

Ion–ion correlations in electrolyte solutions adsorbed in disordered electroneutral charged matrices from replica Ornstein–Zernike equations

The replica Ornstein–Zernike (ROZ) equations, supplemented by the hypernetted chain and mean spherical closures, were solved for an ionic fluid adsorbed in a disordered charged matrix. To obtain the numerical solution of the ROZ equations we performed renormalization of the initial equations. Both the matrix and adsorbed fluid were modeled as charged hard spheres in a dielectric continuum, i.e., in the so-called restricted primitive model. As a result, the pair distribution functions between fluid ions and for fluid-matrix correlations were obtained. Structural properties were studied as a function of the matrix density, the concentration of adsorbed electrolyte and for different prequenching conditions. The isothermal compressibility, excess internal energy, and the chemical potential were calculated and discussed with respect to of the model parameters. Comparison with the Monte Carlo computer simulations of Bratko and Chakraborty [J. Chem. Phys. 104, 7700 (1996)] indicates that the theory yields qualit...

Density functional theory for inhomogeneous associating chain fluids

We propose a nonlocal density functional theory for associating chain molecules. The chains are modeled as tangent spheres, which interact via Lennard-Jones (12,6) attractive interactions. A selected segment contains additional, short-ranged, highly directional interaction sites. The theory incorporates an accurate treatment of the chain molecules via the intramolecular potential formalism and should accurately describe systems with strongly varying external fields, e.g., attractive walls. Within our approach we investigate the structure of the liquid-vapor interface and capillary condensation of a simple model of associating chains with only one associating site placed on the first segment. In general, the properties of inhomogeneous associating chains depend on the association energy. Similar to the bulk systems we find the behavior of associating chains of a given length to be in between that for the nonassociating chains of the same length and that for the nonassociating chains twice as large.

Phase transitions in an associating, network-forming, Lennard-Jones fluid in slit-like pores. II. Extension of the density functional method

We study adsorption of hydrogen-bonded fluids in slit-like pores with strongly attractive walls, in the framework of the four-site associating Lennard-Jones model. The density profiles, as well as the phase behavior, are obtained by using a density functional method. We have found that, at temperatures lower than the critical temperature of the bulk fluid, the confined fluid undergoes one or more layering transitions dependent on the pore width, followed by capillary condensation. Each of the transitions is localized by analyzing the grand thermodynamic potential. The density profiles of nonbonded and differently bonded particles demonstrating changes of the structure of the fluid in the pore along the coexistence are discussed briefly. The critical temperature for capillary condensation is lower for confined fluid, compared with that for the bulk liquid–vapor transition, as expected. However, an increase of the energy of association between fluid species increases the critical temperatures for layering t...